All “so called” bonded phases used in high performance liquid chromatography (HPLC), ultra-performance liquid chromatography (UPLC), and solid phase extraction (SPE) use silica particles as the inert substrate material. The surface of silica particles is heterogeneous, with a variety of different types of silanol groups present on the surface. Although silica can be used as HPLC stationary phase or as an SPE sorbent without further modification, to increase a phase's applicability and adaptability, the surface of the silica material is modified by bonding a wide variety of functional groups to the surface. The nature of the functionality can be non-polar (e.g. an alkyl group), polar (e.g. NH2), ionic (e.g. propylsulphonic acid) or be combined to present a mixed-mode. In general, the hydrophobic nature of silica based sorbents is entirely dependent upon the bonded groups.
These bonded silica sorbent particles are manufactured by reaction of an organosilane with the silica surface. The organosilanes consist of a silicon atom bonded to an organic functional group, for example an octadecyl (C18) group, and one to three chlorines. The two common types of organosilanes are monofunctional organosilanes having one chlorine, and trifunctional organosilane having three chlorines. Monofunctional organosilanes yields a product having a more lightly loaded surface with more active silanol groups than do trifunctionally bonded silicas. The surface with numerous accessible silanol groups provides a polar character to sorbents manufactured using monofunctional silanes (e.g. ISOLUTE MF C18), which can be very useful. However, sorbents manufactured using monofunctional silanes tend to be less stable at the extremes of pH because of the single point of attachment of the silane to the silica particle. Trifunctional bonding chemistry gives rise to somewhat of a “polymeric surface” that exhibits a higher organic functional group loading and fewer silanol groups due to some hydrolysis and condensation between trifunctional silanes.
This state of the art approach to preparing bonded silica particles result in a number of inherent shortcomings. For example, a thin coating of the bonded phase must provide all of the required analyte/sorbent/stationary phase interactions, which imposes a requirement of a high volume of stationary phase loaded into a long column size to achieve a large sample break-through volume, which is the maximum sample volume that can be passed through the column without saturating the bed with the analytes. Often there is an insufficient organic group loading per unit mass of the stationary phase/SPE sorbent to achieve adequate separation or absorption. The state of the art stationary phases display a very narrow range of pH stability, typically, at best, having a robust stationary phase when maintained within a pH range of 2 to 8.
Hence, there remains a need for silica based materials for chromatograph and sorption materials that avoid the shortcomings of surface bonded silica particles that display poor hydrolytic stability due to the ease of eroding a surface functionality from the surface. The new approach eliminate the use of preformed silica particles as the inert surface to graft different alkyl pendant groups via silane surface modification, resulting in surface Si—O—Si—C8/C18 linkage, which is known to possess poor hydrolytic stability. This poor hydrolytic stability of surface Si—O—Si—C8/C18 linkage results in a narrow pH stability (pH 2-8) of the silica based commercial HPLC stationary phases and SPE sorbents. As such, exposing these stationary phases and SPE sorbents beyond the narrow working pH range severely compromise the structural and chemical integrity of these materials.